Marx's admiration for Darwin has never been doubted, yet the relationship of historical materialism to Darwin's theory of evolution by natural selection has been an arena of continuing debate over the last century and more. In this essay I outline some of the main themes in this debate. I begin with a discussion of Darwin's theory, before outlining Marx and Engels' reception of that theory. Third, I discuss the relationship of historical materialism to social evolutionism, as it has been understood by some of the most important Marxists of the last one hundred years. I conclude that the most sophisticated and convincing interpretations of Marx's theory of history have been those that have refused to fall into the alternate traps of either crude evolutionism or voluntarism, but which have synthesized these two elements of Marx's thought into a materially grounded theory of revolutionary action.